1. Field of the Invention
The invention relates to a process and an apparatus for comminuting silicon granules.
2. Background Art
Fluidized bed deposition processes for the production of high-purity silicon granules for the electronics industry or the photovoltaics industry have been described and disclosed in numerous publications. In these processes, silicon particles in a fluidized bed reactor are fluidized by gas and heated to high temperatures. Introducing a silicon-containing gas into the fluidized bed (e.g. silane, tetrachlorosilane or trichlorosilane) leads to a pyrolysis reaction at the particle surface, with elemental silicon being deposited on the particle surface, leading to particle growth. For continuous operation of these processes, relatively large “grown” particles have to be extracted from the fluidized bed as product, and fine particles, known as silicon seed particles, have to be supplied on an ongoing basis. A process of this type is described, for example, by U.S. Pat. No. 3,963,838.
A number of criteria are important for the use of the silicon seed particles in the fluidized bed processes described. First, the silicon seed particles must have a high purity to ensure that the silicon granules produced also meet the requirements of the electronics and photovoltaics industries. In general, metal contamination should amount to less than 100 ppbw, preferably less than 10 ppbw; contamination with dopants boron and phosphorus should be less than 1000 pptw, preferably less than 250 pptw; and contamination by carbon should be less than 1000 ppbw, preferably less than 250 ppbw.
Second, although the size of the silicon seed particles should be smaller than that of the silicon granules, it cannot be too small. This is because particles which are too fine will be discharged from the fluidized bed with the exhaust gas stream and are therefore unsuitable for use as silicon seed particles. The limit for the discharge, depending on operating conditions and processes, is in the range from approx. 50 μm to 250 μm. A fines fraction in the silicon seed particles below this limit leads to silicon losses and to dust being entrained in the exhaust gas systems from the fluidized bed deposition. Finally, the grain size distribution of the silicon seed particles has an effect on the particle population balance of the fluidized bed deposition and therefore also on the grain size distribution of the silicon granules. To achieve a steady-state grain size distribution of the silicon granules which is not excessively wide, it is important for the silicon seed particles to be able to be produced reproducibly with a defined and narrow grain size distribution.
In addition to producing silicon granules, silicon seed particles are also required as a starting material in the photovoltaics and electronics industries, for example, the use of fine-grain high-purity silicon powder for the production of silicon carbide powder as a base material for high-resistance substrate material for electronic devices, described in German laid-open specification DE 19842078A1, and for the production of wafers for photovoltaic applications by melting fine-grain silicon powder on a substrate material, followed by cooling and solidification, as described in U.S. Pat. No. 5,496,416. Hitherto, it has often been impossible, or at least very difficult and/or expensive to obtain corresponding grain size distributions directly from the fluidized bed deposition or by classification from the product of the fluidized bed deposition.
There are several known technologies for producing silicon seed particles. According to U.S. Pat. No. 4,207,360, silicon particles which have been fluidized with inert gas tend to be comminuted in a high-temperature fluidized bed (approx. 1000° C.). In the process, the desired silicon seed particles are formed. The document also mentions the variant of combining this fluidized bed with a fluidized bed for depositing silicon to form an integrated process. Both processes have the drawbacks of extremely high energy consumption which is necessary to heat the fluidized bed, and silicon seed particle grain size distribution and production rate which are very difficult to control.
U.S. Pat. No. 4,314,525 describes a further integrated process in which silicon-containing gases, in particular silanes, are heated to or beyond their decomposition temperature. In the process, the silicon-containing gas breaks down to form extremely fine silicon particles. The particles formed during this homogenous vapor deposition are referred to as nuclei, and can theoretically be used as silicon seed particles. One drawback is that the size of the nuclei is in the nanometer range, and even agglomeration effects can only produce silicon seed particles of a few micrometers. The use of such particles as silicon seed particles in a fluidized bed for the production of silicon granules, typically with a grain size in the range from 50 μm to several millimeters, leads to a large proportion of these tiny silicon seed particles being discharged from the reactor with the gas stream. Additional equipment is required to avoid this.
In addition to these thermal-mechanical or chemical processes, purely mechanical comminution processes are also known for the production of silicon seed particles. According to the Abstract of JP 57-067019 (Shin Etsu Hondatai), silicon seed particles are obtained from silicon granules by the granules being comminuted in a double roll crusher and then fractionated by sieving. Contamination of the silicon seed particles with other elements is prevented by the surface of the rolls being provided with a layer of silicon. The silicon-silicon material pairing between roll and material being milled, however, leads to considerable wear to the layer of silicon on the rolls, and consequently only short machine campaigns are possible before the rolls have to be replaced. A significant improvement with regard to roll wear while at the same time still achieving a low level of contamination of the material being milled is brought about by the use of rolls with a hard metal surface and a modified roll nip geometry, as described in DE 102004048948.
The Abstract of JP 08-109013 describes a further comminution process. According to this document, pre-crushed lumpy silicon can be comminuted in a pinned disk mill to form silicon seed particles. A disadvantage of designs of this type is that it is almost impossible to produce a contamination-free or low-contamination material. Considerable contamination of the milled product is likely. Therefore, downstream wet-chemical cleaning of the surface of the milled product is imperative if this product is to be used to produce high-purity silicon.
U.S. Pat. No. 4,691,866 describes a process in which silicon granules are accelerated to high speeds by means of a gas jet in accordance with the injector principle and propelled onto a stationary obstacle. The impact crushes the particles to form the desired silicon seed particles. To keep contamination at a low level in this process, the obstacle is preferably made from silicon. However, as has already been mentioned in connection with the roll crushing process, the silicon-silicon material pairing leads to considerable wear to the obstacle.
Jet mills for the comminution of very pure materials are described in Fokin, A. P.; Melnikov, V. D.: Grinding Mills In The Production Of Ultrapure Substances—Zhurnal Vses. Khim. Ob-va im. D. I. Mendeleeva; Vol. 33, No. 4, pp. 62-70, 1988. In jet mills, the granular feed material is accelerated by high-speed fluid jets. When these accelerated particles impact on particles with a lower velocity, impact stresses occur, with the particles being broken up according to the impact energy.
A counter-jet mill for producing silicon seed particles is known from Rohatgi, Naresh K.; Silicon Production in a Fluidized Bed Reactor: Final Report—JPL Publication; No. 86-17, 1986. In this process, particles are accelerated by two gas jets and are propelled onto one another so that particles break up. However, the authors mention only a low yield, and consequently the silicon granules have to be milled a number of times.
U.S. Pat. No. 4,424,199 describes a process in which, in addition to the other gas streams, a single high-speed gas jet is used in a fluidized bed for the deposition of silicon, in order for some of the silicon granules in the fluidized bed to comminute to form silicon seed particles. An advantage of this process is that there is no need for any silicon granules to be removed from the deposition reactor, milled and returned, but a disadvantage is that in this case too, the production rate and the resulting grain size distribution of the silicon seed particles are difficult to control. However, controlled operation of fluidized bed deposition is a fundamental requirement. Moreover, the gas jet may have an adverse effect on the deposition process in the fluidized bed. The idea of jet milling within the fluidized bed for silicon deposition is one that is also addressed by S. Lord in U.S. Pat. No. 5,798,137.
The simplest form of a jet mill is the fluidized bed jet mill with a gas jet directed vertically upward. In these jet mills, the gas jet on the one hand accelerates the particles but on the other hand also ensures that the particles are kept suspended in the milling chamber, i.e. are in the fluidized state. It is customary for mills of this type also to be equipped with a classification apparatus which classifies particles discharged with the gas stream, and which recirculates excessively coarse particles to the fluidized bed. A corresponding apparatus is described, for example, in U.S. Pat. No. 4,602,743.
FIG. 1 shows the structure of a conventional fluidized bed jet mill. In an arrangement of this type, the milling gas (1) or the milling gas stream is supplied via a jet nozzle (4) (designed as a simple nozzle or as a Laval nozzle), which is arranged at the base of the milling chamber (5). The feed material (2) is fed to the milling chamber from the side via an inlet (6). A fluidized bed (7), in which the particles accelerated by the gas jet collide with other particles and are thereby comminuted, is formed in the milling chamber from milling gas (1) and particles. The comminuted particles escape from the milling chamber in the upward direction together with the milling gas stream (1) as a joint stream (3). The gas jet is used firstly for acceleration and subsequent comminution of the particles. Secondly, however, the milling gas stream (1) also leads to a classifying effect in the milling chamber. Although the gas jet locally flows into the chamber at a very high velocity, on account of the jet widening and the interaction with the particles, the gas stream is distributed uniformly over the cross section of the milling chamber. If the rate at which a particle drops in the milling chamber is lower than the mean gas velocity in the milling chamber (volumetric flow of the milling gas (1) with respect to the cross section of flow of the milling chamber), the particle is carried out of the milling chamber (5) with the milling gas stream (1).
In gas/particle systems, and in particular in gas classification processes, the particle size whose drop rate in the milling chamber precisely matches the mean prevailing gas velocity for example in a classification apparatus is referred to as the separation grain size. The drop rate of a particle in the milling chamber is directly dependent on its grain size, increases considerably with increasing grain size and can be calculated for example by means of the following formula:18Rews+3Rews1.5+0.3Rews2−Ar=0where
                              Re          ws                =                                            u              ws                        ⁢                          d              p                                v                                    Ar        =                                            (                                                p                  s                                -                                  p                  f                                            )                        ⁢                          gd              p              3                                                          p              f                        ⁢                          v              2                                          whereuws Drop rate of individual particles in the milling chamberdp Particle diameterν Kinematic viscosity of the fluidps,pf Density of the solid or fluid, respectivelyg Gravitational acceleration
Therefore, defining the milling gas stream and milling chamber cross section also defines the separation grain size and therefore the upper limit for the grain size distribution of the milled product. Accordingly, increasing the introduction of energy into the milling chamber by means of a higher gas throughput increases the separation grain size, and therefore also the upper limit, and consequently also the mean grain size of the comminuted particles which are discharged from the milling chamber. At the same time, the solids concentration in the fluidized bed drops. The introduction of energy and the grain size which can be achieved are directly linked to one another.
Jet mills are eminently suitable for the comminution of high-purity silicon granules, because the comminution is effected substantially as a result of the silicon particles colliding with one another. The stressing of components which are in contact with the particles is of only subordinate importance. Moreover, the corresponding components can be lined with a noncontaminating or low-contamination material or may be made entirely from such material. The supply of energy for comminution is effected solely by way of the gas jet. If high purity gases are used for this purpose, the gas jet will also not represent a contamination source. The main drawback of conventional jet mills for the production of silicon seed particles is the fact that the optimum working range of these prior art apparatuses is at milled product grain sizes of approx. 2 μm to 10 μm, i.e. in the range of what is known as ultrafine comminution. In this range, the specific gas consumption is less than 10 kg of gas per kg of solid; Robert H. Perry; Don W. Green: PERRY'S CHEMICAL ENGINEERS' HANDBOOK, 7th Edition,—McGraw-Hill; 1997, Section 20-47. The larger the grain size of feed material and milled product becomes, the less efficiently the mill operates; the specific gas consumption becomes greater and the process correspondingly less economical. It is customary then to switch to other comminution processes, such as for example the roll crushing process referred to above.
U.S. Pat. No. 5,346,141 explains that a significant factor in the efficiency of fluidized bed jet milling for the production of silicon seed particles is the solids concentration in the fluidized bed; the term “solids concentration” is to be understood as meaning the volumetric concentration of the solids particles: a high solids concentration in the region of the jet nozzle leads to a considerable drop in the milling performance, and consequently this patent teaches carrying out the milling in a very dilute fluidized bed with solids concentrations of preferably less than 10% by volume. Nevertheless, the milling performance which can be achieved with this arrangement remains very low and is associated with a high specific energy and/or gas consumption. Example 2 of U.S. Pat. No. 5,346,141 cites a specific nitrogen consumption of 48 kg of nitrogen per kg of solid for the comminution of silicon granules to a mean grain size of 445 μm: 200 liters/minute of nitrogen; 5.2 grams/minute of silicon, i.e. a gas consumption which is approx. five times as high as is otherwise customary in ultrafine jet milling. Moreover, the space-time yield is very low. According to this document, an increase in the solids concentration leads to a further drop in the milling performance.